In recent years, conventional compounded explosives used as blasting agents in hard rock mining, excavation, and the like, have been replaced to a considerable extent by less expensive compositions; that is, the high grade and more costly chemical compositions such as TNT, dynamite, nitroglycerine, nitro-starch and the like have been largely supplanted by compositions based to a large extent on cheap ammonium nitrate, particularly fertilizer grade ammonium nitrate (FGAN). Thus, "ANFO", a mixture of ammonium nitrate (AN) and fuel oil (OF) has been widely used under conditions where it is suitable, e.g. where there are no serious problems with water in boreholes and "slurry", which can vary widely in composition, but usually is based on a saturated aqueous solution of AN with thickeners, gelling agents, particulate or liquid fuels, etc., is widely used where ANFO is less suitable.
The principal reasons for shifting to these newer compositions involve considerations of cost. Fertilizer grade ammonium nitrate is widely available and is relatively cheap; fuel oil, even with energy shortages, is much cheaper than most other fuels that could be used for oxygen balance. Water, as a carrier for the oxidizer salt in slurries, is of course very inexpensive, although some of the other ingredients of slurry, such as gel thickeners, particulate high energy metals, such as aluminum granules, particulate magnesium and the like, used as fuels, usually are not cheap.
These newer blasting agents, however, have their deficiencies. ANFO has low bulk density which limits its blasting energy--also, it has poor water resistance and cannot be used in water-containing boreholes unless it is packaged in water-tight containers. Its low bulk density, usually around 0.85 grams per cc., is often a serious disadvantage. Slurry can be made more dense than ANFO but it becomes increasingly difficult to detonate as density is increased. It usually has to be aerated or "gassed" to make it sufficiently sensitive for detonation by conventional primers or boosters. Apparently, tiny air bubbles act as "hot spots" or activation sites to promote detonation. In addition, when it is to be used in relatively deep columns or boreholes, the hydrostatic head imposed on the lower part of the column increases its specific density and it may fail to detonate, leaving unexploded but dangerous charges or partial charges in areas where further mining or excavation operations must be conducted. Many modifications have been proposed, such as proportional gassing to compensate for the compression of normal aeration, cross-linking agents to prevent bubble migration, as well as gelling agents and thickeners, to stabilize the small bubbles of air or gas which provide sensitive "hot spots" to promote the detonation wave, etc. Coalescence of the very small bubbles into larger ones has a marked desensitizing effect. Various fuels have been used and still others proposed to achieve oxygen balance and to give high energy to the explosive, but these are often expensive. Aluminum particles are very effective as fuels but like other ingredients already mentioned, they add substantially to the cost of slurries.
Various suggestions have been made in the prior art for combining slurries and ANFO in attempts to obtain advantages of both. For example, it has been suggested that AN can be made more water resistant if coated with a "grease", and one proposed method is to dust the AN prills with a soap powder which should react with the oil and provide a water resistant coating (Wilson, U.S. Pat. No. 3,287,189). After such treatment, the coated AN is used in a slurry and the deficiencies of slurry, its need for aeration to make it sensitive, etc. still tend to be present. Egly, U.S. Pat. No. 3,161,551, has proposed use of a water-in-oil emulsifier to provide fuel and completely fill the voids in the prilled AN, thereby improving its resistance to moisture, but compositions of this type have been found not to be reliably detonable without being aerated or gassed (See Bluhm U.S. Pat. No. 3,447,978).
Since dense slurries are insensitive and hard or impossible to detonate it has often been proposed to aerate them by heating air into them--see for example, Cattermole, U.S. Pat. No. Re. 28,060. This reference suggests also the use of a water-in-oil emulsion, comprising AN in water, oil and a suitable emulsifier for improving water resistance. To aerate, for sensitivity, the addition of glass bubbles or micro balloons, is suggested. The reference suggests guar gum is to be eliminated, in view of its cost. Wade, U.S. Pat. No. 3,765,964, and Bluhm, U.S. Pat. No. 3,447,978, have somewhat similar suggestions, in part. The latter is of some interest because the water-in-oil emulsion per se there described is rather similar to a composition preferred for the present invention as one of its two major constituents. Bluhm describes the preparation of a water-in-oil emulsion to serve, as such, for the entire explosive composition. It consists of a major proportion of AN, water, a "carbonaceous" fuel which comprises or consists of a special wax, and a water-in-oil emulsifier. He also suggests that the AN may be supplemented by other powerful oxidizers, such as sodium nitrate (SN). His composition must be compounded to include considerable quantities of air, as occluded gas, in volume proportions as high as 37% or more. Small "microspheres", preferably tiny glass bubbles, have been proposed by many as gassing agents. The use of compressible gas-containing explosives in deep boreholes, gives rise to problems already mentioned. The glass bubbles are costly and often not very helpful. However, they are sometimes useful.
Still others in the prior art have suggested various other schemes, procedures and additives to provide the aeration, considered so necessary for density control because sensitivity or detonability is so closely related to density. While such compositions become more sensitive to detonation as their density decreases, other things being equal, they also lose strength in proportion to the reduction in their bulk density in the borehole.
As noted above, many slurries require guar gum or equivalent costly thickener to hold the ingredients together and to impart water resistance. They frequently require also the use of supplemental thickeners or cross-linking agents to hold the gas bubbles in situ or prevent them from coalescing and losing their effectiveness. All these ingredients, even in very small proportions, add substantially to the cost of the finished explosive. In addition, they often cause instability problems. An important object of the present invention is to keep the cost down by avoiding the use of all such costly ingredients, making use of the natural porosity or interstices in particulate salts such as AN. A further object is to simplify the procedures required for making the blasting composition, thereby further reducing costs.
In general terms, the present invention relates to a novel blasting composition which is finally put together from two main and relatively simple constituents, namely, (1) a water-in-oil emulsion and (2) ANFO or AN. Where ANFO is used, not AN, each of these two constituents, i.e., emulsion and ANFO is approximately oxygen-balanced in the preferred procedure, before they are combined, so that the resulting product will necessarily be oxygen balanced, at least approximately. In some cases, however, one constituent may be over-balanced and the other underbalanced, as in the case where the water-in-oil emulsion contains most or all the oil and the other ingredient is primarily the oxidizer salt (AN, or AN plus other salts of generally similar properties), without fuel, or with inadequate fuel for balance.
The two main constituents, i.e., emulsion (or slurry) and essentially solid particles are combined by very simple procedures, preferably just prior to insertion into the blasting site (borehole). Conventional apparatus, such as augur type delivery means may be used to assist in mixing the emulsion into the AN or ANFO. To a large degree, the simplicity and economy of ANFO are retained while a much denser and more efficient but still detonable blasting agent is produced.
Further objects of the present invention, therefore, include the compounding of inexpensive ingredients by simple operations to obtain an improved explosive composition of greater bulk density but adequate sensitivity for good detonation. This avoids the necessity of gassing or aerating and thereby tends to eliminate difficulties which arise in connection with aerated slurries of the prior art. Advantage is taken of the structural properties of AN, or of ANFO, to provide essentially a non-compressible aeration.